Throw an RPC error when calling methods on a disposed connection.

This is consistent with the way we handle outstanding requests when the
service is disposed. Note that some of the devtools code in flutter
expects and handles this RPC error in some cases.

Change-Id: I56aae14f898b6d62b77d10ef50dec63ae96d507d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/367821
Reviewed-by: Ben Konyi <[email protected]>
Commit-Queue: Jackson Gardner <[email protected]>

Author: eyebrowsoffire

pkg/vm_service/lib/src/vm_service.dart

@@ -1769,6 +1769,13 @@ class VmService {
   }
 
   Future<T> _call<T>(String method, [Map args = const {}]) {
+    if (_disposed) {
+      throw RPCError(
+        method,
+        RPCErrorKind.kServerError.code,
+        'Service connection disposed',
+      );
+    }
     return wrapFuture<T>(
       method,
       () {